1. Field of the Invention
The present invention generally relates to a method for manufacturing semiconductor packages, and more particularly, for manufacturing a chip scale package ("CSP").
2. Description of the Related Art
Electronic industry trends, such as miniaturization and multifunctionalization of electronic devices, have resulted in a relatively new semiconductor package called a Ball Grid Array ("BGA") package. When compared to conventional plastic packages, the BGA package has a higher surface-mounting density and superior electrical capabilities. In some respects, however, the BGA package is not as reliable as some conventional packages. Unlike a conventional plastic package that uses moisture-resistant lead frames, the BGA package uses a printed circuit board that is more moisture-prone. Another disadvantage of the BGA package is that space must be reserved on the board for mounting the semiconductor chip. In view of the foregoing drawbacks, a Chip Scale Package ("CSP") has been proposed.
Many companies in the United States, Japan and Korea have developed or manufactured various types of CSPs. One such type of CSP is called the Fine Pitch BGA ("FPBGA") package. The .mu.-BGA package, developed by Tessera in the U.S., is an example of a FPBGA package. The FPBGA packages employ a thin and flexible circuit board, such as a tape wiring board. The flexible circuit board includes beam leads that connect to bonding pads of a semiconductor chip through windows formed in the board.
FIG. 1 is a cross-sectional view of a conventional .mu.-BGA package 200. Referring to FIG. 1, a tape wiring board 120 includes a polyimide tape 124 having top and bottom surfaces. Copper (Cu) traces 130 are formed on the bottom surface of the polyimide tape 124. Beam leads 160 extend from the Cu traces 130. An elastomer layer 150 is interposed between the wiring board 120 and a semiconductor chip 110. Beam leads 160, bonded to bonding pads 112 on the semiconductor chip 110, electrically connect bonding pads 112 to respective solder bumps 168 via the Cu traces 130 and the solder ball mounting pads 136.
The solder ball mounting pads 136 are portions of the Cu traces 130 that are exposed through connection holes 123. An encapsulant 189 encapsulates the bonding area between the bonding pads 112 and the beam leads 160 to protect the area from external environmental stresses. The beam leads 160 also comprise portions of the Cu traces 138 that bond to the bonding pads 112 on the semiconductor chip 110. The beam leads 160 are plated with gold (Au) to improve the bonding quality between the beam leads 160 and the bonding pads 112. Another Au layer 164 is plated on the solder ball mounting pads 136, and the solder balls 168 are attached to the Au layer 164 on the solder ball mounting pads 136. The solder balls 168 are typically a 63% tin (Sn)-37% lead (Pb) alloy.
FIG. 2 depicts the .mu.-BGA package 200 of FIG. 1 mounted on a main board 170. The solder bumps 168 of the package 200 are soldered to pads 172 on the board 170, typically, in an infrared reflow soldering process in which the soldering process occurs at a maximum temperature of about 220 to 230.degree. C. During the soldering process, Au atoms from the Au layer 164 diffuse into the solder balls and form an intermetallic compound 168a with the Sn and Pb atoms of the solder balls. The intermetallic compound 168a migrates to the outer surface of the solder bumps 168 and deteriorates the solderability between the solder bumps 168 and the pads 172 on the main board 170. It would be desirable if the formation of this undesirable intermetallic compound 168a could be eliminated.